The invention relates generally to frequency domain high-pass electronic filters and relates more particularly to precision switched capacitor filters for integrated circuits.
A known basic high-pass frequency domain filter is shown in FIG. 1. It includes a signal input terminal, a d.c. (direct current) blocking capacitor C, a resistor R, and a signal output terminal such as would typically be connected to the summing node of a negative feedback amplifier. The reference symbol C, as is used herein with different subscripts to identify particular capacitors, is also used to denote the capacitance value of that capacitor.
In very large scale integrated MOS (metal-oxide-silicon) circuits, it is generally not feasible to provide either a precision resistor for R or to provide a capacitor whose value tracks the inverse of the resistor value over variations in the manufacturing process. The resistor R can, however, be replaced by a capacitor C.sub.2, which is switched by a pair of break-before-make electronic toggle switches S.sub.1, S.sub.2 operated by non-overlapping switching pulse trains .phi..sub.1, .phi..sub.2 to result in the known switched capacitor filter structure of FIG. 2. The value of the capacitor C.sub.2 corresponding to the resistance value of resistor R is 1/C.sub.2 f.sub.s, where f.sub.s is the frequency of the switching pulse trains. It is noted that larger C.sub.1 capacitance values correspond to lower break frequencies for such a filter. The break frequency of a filter is that frequency associated with the knee of the transmission characteristic curve for that filter.
One problem with the switched capacitor filter structure of the type shown in FIG. 2 is that for a given capacitor value C.sub.2, as required for output drive, and a given switching pulse rate for the electronic switches S.sub.1, S.sub.2, a relatively large value is required for the capacitance C.sub.1 in order to obtain relatively low break frequency capability. This is because a lower limit on the value of the capacitance C.sub.2 is determined by the nature of the load circuitry. Large capacitance value elements require additional area on the integrated circuit chip. Furthermore, requiring the signal source to supply all of the current required to charge C.sub.2 may also lead to significant additional cost.
Another problem relates to a parasitic capacitance C.sub.X, shown in phantom lines in FIG. 2, which is present on the switch S.sub.1 side of the capacitance of C.sub.2 in the prior art circuit. This additional switched capacitance is troublesome in that it is generally voltage-dependent, and introduces non-linear transmission loss which may be significant.
While a lower break frequency capability could also be obtained for the prior art structure by lowering the frequency of the switching pulses for the switches S.sub.1, S.sub.2, this would cause problems in realizing other circuit functions on the chip, assuming that it is the main clock frequency that is lowered; it might also create new antialiasing problems if the switching pulses represent a further count-down from the main clock.